1. Cooling System of Nuclear Reactor
Nuclear Reactors under construction are roughly classified into: fast reactors using fast neutrons; and thermal-neutron reactors using thermal neutrons. Many of the fast reactors use uranium (U) and/or plutonium (Pu) as fuel, and metallic sodium (Na) as a primary coolant. Existing power generating systems have used a steam turbine, and thus need a heat exchanger for transferring heat between metallic Na and water (H2O). If metallic Na contacts H2O, metallic Na will reacts with H2O, and an explosive hydrogen gas will be produced. This has been a problem. With danger due to the reaction between metallic Na and H2O taken into consideration, the number of actually-constructed fast reactors has not been so large. The followings are among methods of constructing a cooling system without using the combination of metallic Na and H2O.
One method is to not use H2O as the secondary coolant although metallic Na is used as the primary coolant.
The most promising candidate as the secondary coolant is carbon dioxide (CO2) gas. Research and development on the application of supercritical carbon dioxide gas to a turbine has been underway, and an actual turbine is currently about to start its production. A supercritical carbon dioxide gas turbine with a power generation capacity of up to 10 MW can be produced. Patent Literature 1 relates to a small-scale nuclear power generation system which uses metallic Na as the primary coolant and supercritical carbon dioxide gas as the secondary coolant. The nuclear power generation system, however, is considered as having a positive void reactivity, and having difficulty in maintainability during fuel replacement and the like due to unclearness of metallic Na.
Another method is to use Pb—Bi or Pb alone as the primary coolant while using H2O as the secondary coolant. Research and development on a Pb—Bi reactor is underway in Russia, and the reactor uses a Pb—Bi eutectic alloy (45% of Pb vs. 55% of Bi). Yet another method is to use Sn. Focusing on Pb—Bi, effectiveness and the like will be hereinbelow discussed.
Advantages of Pb—Bi coolant can be cited as follows.
(1) The Pb—Bi coolant requires no intermediate cooling system. Since heavy metals have low reactivity with water, facilities for preventing fire from starting with combustion due to leakage of the coolant is also not needed.
(2) Lead has a boiling point of 1,737° C., and the lead-bismuth eutectic alloy has a boiling point of 1,670° C. Their boiling points are higher than that of Na. Use of the high boiling points of the respective coolants makes it possible to obtain higher safety.(3) The neutron absorption cross section of Pb—Bi is smaller than that of Na, and the scattering cross section of Pb—Bi is larger than that of Na. Excellent proliferation and minor actinide (MA) burnup can be expected from the Pb—Bi coolant.(4) The coolant density of the Pb—Bi coolant is almost equal to that of mixed oxide fuel (MOX fuel). If the fuel melts, the molten fuel is less likely to accumulate in the bottom portion of the reactor vessel. Accordingly, it can be expected that the Pb—Bi reactor makes the influence of construction of logic for preventing re-criticality upon the reactor core designing smaller than the Na reactor.(5) In metal fuel of a Pu—U—Zr ternary alloy, noble gas components generated under neutron irradiation are homogeneously distributed in the metal structure. Against this background, there is a report that the metal thermal expansion coefficient is larger by three digits or more than in a case where no noble gas components are generated (Non Patent Literature 1). From this result, a phenomenon that like the specific weight of the MOX fuel, the specific weight of the metal fuel of the ternary alloy is lighter than that of the coolant can be expected to take place.
On the other hand, disadvantages of Pb—Bi coolant can be cited as follows.
(1) The heavy metals contained in Pb—Bi coolant are highly corrosive. This corrosiveness narrowly limits selection of structural materials.
(2) Pb—Bi coolant has a greater specific weight than Na coolant. It is considered that Pb—Bi coolant corrodes structural material more than Na coolant when Pb—Bi coolant and Na coolant flow al the same rate. This limits the linear flow rate of Pb—Bi coolant strictly, and decreases design freedom.(3) When Pb—Bi coolant is irradiated with neutrons, volatile polonium (Po210) is produced. This requires hermetic management of the cover gas system covering the coolant, and the fuel handling system. This also makes maintenance work difficult.(4) The greater specific weight of the coolant requires a larger pump shaft power. In addition, “Torricelli's vacuum” causes a problem of decreasing design freedom because of things such as impossibility of employing the top entry method for the main cooling system or the decay heat removal system.(5) An increase in weight due to the greater specific weight of the coolant and greater thickness require reconsideration of supporting structures for the vessels and pipes, and more attention to earthquake-resistant design.
In a case where a well-proven steam turbine is used, a fast-neutron reactor can be operated using Pb—Bi or Sn, a liquid at high temperature, instead of metallic Na. Research and development on fast-neutron reactors using Pb—Bi as the primary coolant have been conducted widely. Use of Pb—Bi as the primary coolant is advantageous in making it possible to use a well-proven steam turbine. On the other hand, heavy metals, such as Pb—Bi, involve a risk of corrosively damaging structural materials at high temperature. This poses a serious practical problem (Non Patent Literature 2).
Meanwhile, the light-water reactor is a typical type of thermal-neutron reactor. There are varieties of light-water reactors: the pressurized water reactor and the boiling water reactor. Technologies for the pressurized water reactor and the boiling water reactor are almost established. In the past, there were carbon dioxide gas-cooled reactors which used carbon as a moderator and carbon dioxide gas as coolant. They have been replaced with light-water reactors. Light-water reactors use oxide fuel, and control a degree of combustion of nuclear fuel basically using a control rod. Spent nuclear fuel is subjected to wet reprocessing.
2. Nuclide Separation and Annihilation Treatment of Radionuclides
2-1. Radionuclides Generated in Nuclear Power Generation and their Properties.
Main nuclear reactions in a nuclear reactor are divided roughly into: a reaction in which other radioactive nuclides (fission products) are produced with neutron irradiation to fissile U235 and Pu239, and a reaction in which irradiation to fissionable transuranic nuclides such as U238 transmute into Pu or other transuranic nuclides by absorbing neutron. Table 1, using Non Patent Literature 3 as a reference and in a summarized form, shows nuclides which have the longest life, the highest amount of radiation and the highest toxicity among the radionuclides contained in spent nuclear fuel to be discharged from a nuclear reactor. As learned from Table 1, spent nuclear fuel is classified roughly into transuranic nuclides (TRUs) and nuclear fission products (FPs). Furthermore, TRUs include minor actinides (MAs). Each MA has a long-life and a large amount of radiation, emits a neutron, and generates heat. From a different viewpoint, MAs are effective as an energy source. MAs basically transform by α-decay, and emit γ-ray after α-decay. Thereafter, MAs repeat β-decay and γ-decay, and eventually transmute into stable nuclides. Neutron bombardment is effective to stabilize MAs. In addition to MAs, FPs which have relative long life and high toxicity are contained in spent nuclear fuel. Many of FPs tend to transmute into stable nuclei through β-decay and γ-decay.
TABLE 1Main Radioactive Elements in Spent FuelDoseNeutronConversionHeatGamma-rayBeamFactorContentGeneratedintensityIntensityNuclideHalf-life(μSv/kBq)(per ton)(W/g)(γ/g · s)(n/g · s)TransuranicPu23887.7years2300.3kg0.563.00E+0836000ElementPu23924000years2508kg0.0021.10E+0696(TRU)Pu2406848years2503kg0.0073.90E+061300Pu24114.3years4.81kg0.00041.20E+071.23MinorNp237214 millionyears1100.6kg0.00027.30E+060.9ActinideAm241432years2000.4kg0.114.90E+107000(MA)Am2437370years2000.2kg0.0075.50E+09540Cm24418.1years12060g2.88.90E+081.20E+07Nuclear FissionSe79295000years2.96gProduct (FP)Sr9028.8years280.6kgZr93153 millionyears1.11kgTc99211000years0.641kgPd107650 millionyears0.0370.3kgSn12610 millionyears4.730gI1291570 millionyears1100.2kgCs135230 millionyears20.4kgCs13730.1years131.5kg
For example, in a case where U235 fissions, approximately 80 types of nuclear fission products are produced. Their mass numbers widely range from 72 to 160. The distribution of their mass numbers is saddle-shaped with one peak around the mass number of 90 and the other peak around the mass number of 140. The decay processes of these fission products are divided roughly into two types: α-decay and β-decay. Usually, γ-decay occurs after α-decay or β-decay. Target nuclear fission products of the present invention are as follows.
(1) Minor Actinide Elements with Long Half-Life
There has been a report that these nuclides undergo transmutation by α-decay, emit neutrons, β-decay, and then emit γ-ray into stable elements. To put it specifically, Np237, Am241, Am243, Cm244 and the like are considered as problematic, as shown in Table 1. These nuclides have long half-life, and tend to generate much heat. These nuclides need measures to counter strong emission of γ-rays and neutron beams. Among these minor actinide nuclides (MAs), Cm244 shows a strong tendency to fission spontaneously. A measure to counter neutrons generated by this spontaneous nuclear fission needs to be taken in order to reduce radionuclides, such as MAs, in a nuclear reactor.
(2) Radioisotope Elements with Relatively Short Life
The decay rates of radioactive nuclides, not the minor actinide elements, are determined mainly by their β-decay processes. Immediately after β-decay, γ-decay occurs. Since this β-decay is a forbidden transition from a viewpoint of quantum mechanics, transition probability is low, and the γ-decay rate is accordingly small.
2-2. Effects Expected from Nuclide Separation and Reducing Processes
High-level radioactive waste generated by operation of a nuclear power plant includes more nuclear fission products, which are more radioactive and have higher heat quantity but have shorter half-life, and more nuclear fission products and transuranic elements, which are less radioactive but have longer half-life, than low-level radioactive waste generated by the operation of the nuclear power plant. The existence of nuclear fission products and transuranic elements with longer half-life makes it impossible to secure safety by the “managed disposal” which reduces management on a step-by-step basis in response to a decrease in the concentration of radionuclides, and completes the management several years later. Against this background, safe burial disposal (geological disposal) of high-level radioactive waste in a deep stable geologic formation away from the human living environment is expected to be performed for the purpose of preventing the living environment from being significantly affected by the radioactive waste. It is difficult, however, to secure a place for the burial disposal. For this reason, a technique of reducing radioactive waste is needed.
2-3. Nuclide Separation Technique
As discussed above, high-level radioactive waste includes: nuclear fission products, which are more radioactive and have higher heat quantity but have shorter half-life, and nuclear fission products which are less radioactive but have longer half-life like actinide elements. The technique of separating nuclear fission products with longer half-life from nuclear fission products with shorter half-life is based on the reprocessing technology of extracting U and Pu from spent fuel. Some different methods have been also under examination. These methods are divided roughly into: an advanced wet reprocessing method using water and an organic solvent as solvent; and a dry reprocessing method using molten chloride and cadmium (Cd) liquid metal as solvent. Both are currently adopted as reprocessing methods in Japan. The wet reprocessing method is used mainly for fuel using metal oxide. Application of the wet reprocessing method makes it possible to produce nuclear weapons by increasing the purity of U or Pu. On the other hand, the dry reprocessing is suitable for metal fuel. It is said that the dry reprocessing method is not suitable to produce nuclear weapon since the concentration of impurities in radioisotope nuclides. The dry reprocessing method extracts U235 and Pu239 by electrolytic refining, but it is impossible to avoid mixture of minor actinide nuclides as impurities since the minor actinide nuclides are close to U235 nuclide and Pu239 nuclide. Patent Literature 2 has disclosed a method of improving accuracy of the electrolytic refining. On the other hand, it is easy to separate radioactive nuclear fission products (FPs) having relatively small mass numbers.
2-4. Decay Rate Acceleration (Reducing Process) Techniques
There are several types of decay rate acceleration (reducing process) techniques as follows,
(Acceleration of Decay Rate Using Neutrons)
Neutrons are divided roughly into fast neutrons with energy of 0.5 MeV or more, and thermal neutrons with energy of 0.5 MeV or less. Although there is a case where thermal neutrons are further divided into groups, the present invention will refer to neutrons with energy of 0.5 MeV or less as thermal neutrons.
The minor actinide (MA) elements shown in Table 1 are materials which, like U and Pu, can be transmuted by neutron capture. With this taken into consideration, one may consider that the most effective way of transmuting minor actinide elements into other stable elements is to cause a nuclear fission reaction using a nuclear reactor. A light-water reactor, a fast-neutron reactor and a proton accelerator-driven subcritical fast-neutron reactor are considered as being usable as the nuclear reactor suitable for the above purpose. As shown in FIG. 1 (Patent Literature 4), minor actinide elements are characterized as capturing fast neutrons with higher energy more efficiently, or as being more efficiently made to undergo a nuclear fission reaction by fast neutrons with higher energy. Furthermore, the reducing process using a nuclear reactor can be considered as using minor actinide elements as fuel for electric power generation.
Nuclear fission products include long half-life nuclides such as iodine (I)-129 (not included in glass-solidified matter) and technetium (Tc)-99. For these nuclides, use of neutron capture reaction which makes a nuclide capture a neutron and transmute into another nuclide, may be considered. In this case, a scheme of sufficiently moderating a neutron is needed before the neutrons are captured by radioactive nuclides in a fast-neutron reactor, in addition, photonuclear reaction which makes a nuclear fission product absorb γ-ray and transmute into another nuclide may be considered as well. Thereby, for example, cesium (Cs)-137 can be transmuted into barium (Ba)-136, a stable nuclide. Use of an accelerator to cause this react has been planned.
(Method of Using Elementary Particle)
In addition to the above-discussed methods, there is a method using an elementary particle. Jere H. Jenkins et al. have reported that fluctuations in decay rate of a radionuclide are correlated with neutrinos which reach the earth from the sun (Non Patent Literature 5). To put it specifically, they have reported that: the rate of β-decay of Si32 fluctuates in correlation with neutrino flux; and the rate of α-decay of Ra226 also fluctuates in correlation with neutrino flux.
Furthermore, S. Abe et al. have reported that: muons accelerate the β-decay rate; and this acceleration would be effective to achieve a process of reducing radioactive reactor waste in the future (Non Patent Document 6). Absorption of a muon by a nucleus transmutes the element into a new isotopic nuclide. However, a large number of muons need to be produced in order to reduce a large amount of radioactive waste. They have reported that a huge accelerator was used to generate muons and elementary particles as their source.
(Another Decay Rate Acceleration Using Field: β-Decay Nuclides)
β-decay is divided into Pi decay with electron emission and β+ decay with positron emission. Before the 1990s, it was said that the β-decay rate was not changed by external conditions. In addition, long-life β-decay transition is basically a forbidden transition from a viewpoint of quantum mechanics. Nevertheless, it has been proposed that: the forbidden transition has no absolute significance; and the decay rate can be accelerated by applying a strong low-frequency electromagnetic field, which can give angular momentum enough to break quantum mechanical selection rules, to nuclei. For the purpose of changing the decay rate of a nucleus, the electronic state in the nucleus needs to be changed by the electromagnetic field. H. R. Reiss et al. have proposed a method of accelerating the β-decay rate in a report (Non Patent Literature 7) given below.
Non Patent Literature 7 has summarized a basic theory on a basic physical phenomenon caused by strong interaction between a low-frequency electromagnetic field and a nucleus, particularly on the β-decay rate. In order to incorporate an allowed transition to a basically forbidden transition, the perturbation theory in quantum mechanics was applied to the interaction between a nucleus and an electromagnetic field in the beginning. For example, using an interaction Hamiltonian λH, a Hamiltonian HT representing a state of a nucleus is expressed withHT=H0+λH  (Equation 1).
When the perturbation term is effective, there are multiple energy levels, and the allowed transition is partially mixed.
Another method is to combine the Hamiltonian HT with a non-perturbation term representing a completely different mechanism. This method is achieved, for example, by: putting an atom into a plasma state; and thereby reducing the shielding effect provided by the electrons around the nucleus. The reduction in the shielding effect makes it possible for an eternal field to directly affect the nucleus. Furthermore, when the nucleus itself is unstable due to its decay or the like, the nucleus is more likely to be affected by the external field.
As a specific method, a possibility of accelerating the β− decay rate, which can be handled as a perturbation term, was measured. It was confirmed that the β− decay rate increased by 10−4 or more when Cs137, a radioactive element, was placed in a cavity resonator at a radio frequency of 1 kHz to 50 MHz. In addition, it has been recently reported that the decay rate was able to be increased by irradiation of a MeV-energy pulse laser beam (Non Patent Literature 8). For example, it has been reported that irradiation of a pulse with a frequency of 1 kHz or less in several femtoseconds (10−15 seconds) was promising (Non Patent Literature 8). Based on these results, in principle, the rate of decay of a radioactive element now can be changed by applying a low-frequency electromagnetic field to the radioactive element. This is because the application of the low-frequency electromagnetic field makes it possible to reduce the shielding effect of the electrons around the nucleus.
Meanwhile, with regard to strange effects related to rotating magnetic fields, M. Pitkanen has reported a phenomenon caused based on topological geometro-dynamics (TGD) (Non Patent Literature 9). An electric field is induced by the Lorentz force when a magnetic field is rotated. M. Pitkanen has reported the likelihood that the reduction in the shielding effect of the electrons around the nucleus using such electric and magnetic fields makes it possible to accelerate the rate β− decay rate using the external field.